Calculating your air conditioning requirements

Heat Load

The amount of heat generated is known as the heat gain or heat load. Heat is measured in either British Thermal Units (BTU) or Kilowatts (KW). 1KW is equivalent to 3412BTUs.

The heat load depends on a number of factors, by taking into account those that apply in your circumstances and adding them together a reasonably accurate measure of the total heat can be calculated.

Factors include:

The floor area of the room

The size and position of windows, and whether they have blinds or shades

The number of room occupants (if any)

The heat generated by equipment

The heat generated by lighting

Floor Area of Room

The amount of cooling required depends on the area of the room. To calculate the area in square metres:

Room Area BTU = Length (m) x Width (m) x 337

Window Size and Position

If, your room has no windows, you can ignore this part of the calculation. If, however there are windows you need to take the size and orientation into account.

South Window BTU = South Facing window Length (m) x Width (m) x 870

North Window BTU = North Facing windows Length (m) x Width (m) x 165

If there are no blinds on the windows multiply the result(s) by 1.5.

Add together all the BTUs for the windows.

Windows BTU = South Window(s) BTU + North Window(s) BTU

Occupants

You will have to take that into account people who normally working in the space. The heat output is around 400 BTU per person.

Total Occupant BTU = Number of occupants x 400

Equipment

Clearly most heat in a room is generated by the equipment. This is trickier to calculate that you might think. The wattage on equipment is the maximum power consumption rating, the actual power consumed may be less. However it is probably safer to overestimate the wattage than underestimate it.

Equipment BTU = Total wattage for all equipment x 3.5

Lighting

Take the total wattage of the lighting and multiply by 4.25.

Lighting BTU = Total wattage for all lighting x 4.25

Total Cooling Required

Add all the BTUs together.

Total Heat Load = Room Area BTU + Windows BTU + Total Occupant BTU + Equipment BTU + Lighting BTU

This is the amount of cooling required so you need one or more air conditioning units to handle that amount of heat.

Disclaimer: These calculations are intended as rough guide only. Complete accuracy cannot be guaranteed

Ward Rg